In recent, with a progress of miniaturization of DRAMs (Dynamic Random Access Memories) a capacitor used in the DRAMs has a three-dimensional cylindrical structure in order to obtain a greater capacity. More specifically, the capacitor is formed by covering a surface of a lower electrode formed in a three-dimensional structure with a capacitor insulation film and by covering a surface of the capacitor insulation film with an upper electrode. In the step of forming the upper electrode, a gap is formed between adjacent capacitors. Therefore, a boron-doped silicon germanium (B—SiGe) film, which has a high conductivity and can be formed at a low-temperature, is used as a conductive layer to fill the gap.
The B—SiGe film is formed by a CVD (Chemical Vapor Deposition) method. At this time, if the B—SiGe film is formed on the upper electrode, uniformity of a film thickness distribution deteriorates significantly. Thus, in some related arts, a surface modification layer (seed layer) is formed on the upper electrode by adsorbing substances obtained by a thermal decomposition of monosilane to a surface of the upper electrode, and the B—SiGe film is formed on the surface modification layer.
In some other related arts, a germanium film is formed after a sufficient amount of boron-doped silicon film is coated on a base film, thereby growing germanium uniformly.
However, since a surface area of the B—SiGe film tends to increase with a necessity of further miniaturization of DRAMs, it is required to further suppress variation in film thickness depending on a state of the base film. Thus, a seed layer formed of the decomposed substances of monosilane as described above cannot provide a sufficient seed effect. Also, it is required to further lower a film forming temperature of the B—SiGe film.
In addition, the B—SiGe film used for DRAMs needs to be in a polycrystalline state even when the film is formed through a low-temperature film formation. However, in a case of forming the B—SiGe film after coating the base film with the boron-doped silicon film as described above, the B—SiGe film becomes an amorphous film.